cmd/go: fix build in airplane mode

LGTM=iant
R=golang-codereviews, adg, iant
CC=golang-codereviews
https://golang.org/cl/122190043

src/cmd/go/vcs_test.go

@@ -12,6 +12,9 @@ import (
 // Test that RepoRootForImportPath creates the correct RepoRoot for a given importPath.
 // TODO(cmang): Add tests for SVN and BZR.
 func TestRepoRootForImportPath(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping test to avoid external network")
+	}
 	switch runtime.GOOS {
 	case "nacl", "android":
 		t.Skipf("no networking available on %s", runtime.GOOS)